1. Field of the Invention
The present invention relates to alignment layers for liquid crystal display devices (hereinafter abbreviated LCD). More particularly, it relates to a photo-alignment material having a photo-reactive functional group of ethenyl at a main-chain, and to the use of that material in liquid crystal displays.
2. Background of the Related Art
Flat panel LCD devices have become widely used as displays for mobile terminals, notebook computers, office equipment, video equipment, and the like. This is because flat panel LCD devices have advantages of small-size, lightweight, and low power consumption.
In general, an LCD includes a pair of substrates that are separated by a predetermined interval, and an interposed liquid crystal.
An LCD has numerous functional requirements, including light transmission characteristics, operational response time, viewing angle, and contrast. Many of those requirements are impacted by the alignment characteristics of the liquid crystal molecules in the LCD. Indeed, uniformly aligned liquid crystal molecules are important to the electro-optical characteristics of LCDs.
The alignment characteristics of LCDs are results of an alignment layer. Generally, a rubbing technique has been used to form that layer. In that technique, a special cloth is rubbed over a substrate to form the alignment layer. While the rubbing technique is a simple process, it has problems. For example, various process variables related to rubbing are difficult to accurately control. Furthermore, dust adsorption, unwanted scratches generated by the rubbing, and damage to thin film transistors caused by static electricity can also result from the rubbing. Such problems reduce the manufacturing yield and the performance of LCDs.
Because of the forgoing problems, significant effort has been expended in developing alignment techniques that do not use mechanical rubbing. In particular, photo-alignment methods could solve the static electricity and dust problems, as well as improve the viewing angle.
Photo-alignment methods include photo-decomposition, photo-polymerization, and photo-isomerization. In these methods, optical anisotropy is brought about in a polymer layer by inducing a photo-reaction after most of the molecules facing a polarizing direction in the disorderly-aligned polymer molecules have absorbed light.
In the photo-decomposition method, liquid crystals are arranged by inducing optical anisotropy using a photo-decomposition reaction that selectively breaks partial bonds of the molecules in a specific direction by the application of linearly-polarized ultraviolet rays to a polymer layer consisting of a photo-alignment material. The material typically used for this method is polyimide. Although polyimide requires the application of ultraviolet rays for a relatively long time to induce liquid crystal alignment, a polyimide alignment layer formed by photo-decomposition has a relatively-high thermal stability as compared to other photo-alignment layers fabricated by other methods.
In the photo-polymerization method, liquid crystals are arranged by polymerizing the molecules in a specific direction by applying linearly-polarized rays to a polymer layer where polymerization is to occur.
In the photo-isomerization method, cis/trans isomers are formed by a polarized light. Thus, liquid crystals are aligned by the direction generated from the transformation of the produced isomers. Although the alignment direction is reversibly controlled by applying light of a specific wave length, it is difficult to give a pre-tilt angle to a liquid crystal as well as to maintain the stability of the liquid crystal.
The chemical structure of photo-alignment materials is mainly divided into two categories: a main chain, and side chains including photo-sensitive groups such as an alkyl group, an ethenyl group and the like. The main chain makes liquid crystal molecules arrange to face a predetermined direction, while the side chains form a pre-tilt angle.
Photo-reactions take place at side chains having photosensitive groups when light is irradiated onto them. Thus, liquid crystal alignment depends on the side chains. Therefore, liquid crystal alignment of the photo-alignment material is controlled by the side chains, which include hydrocarbon branches such as alkyl, ethenyl and the like.
Unfortunately, related art photo-alignment materials, and LCDs using the same, have problems. As noted, the photo-alignment of a photo-alignment layer according to the related art is controlled by the side chains. As the side chains are flexible and fragile, that alignment tends to be easily broken by thermal, physical, electrical, and photo shocks. Furthermore, the alignment tends to be hard to restore. Additionally, the photo-alignment layer used in the photo-decomposition method has such poor photo-sensitivity that relatively high light intensity is required to break the predetermined bonds of the side chain or main chain. Finally, in general, with related art photo-alignment layers the actual liquid crystal alignment is relatively poor.